The invention generally relates to controlling a dynamic signal range in an optical time domain reflectometry system.
It is often desirable to obtain a temperature versus depth profile of a region of interest. For applications involving a well, the temperature versus depth profile may be used for such purposes as monitoring hydraulic fracturing in the well (as an example). An optical fiber is one type of sensor which may be used to acquire the temperature versus depth distribution. More specifically, the optical fiber may be part of an optical time domain reflectometry (OTDR) system, a system that typically includes a reflectometer that launches an optical pulse into the optical fiber and tracks backscatter radiation as a function of the elapsed time since the launching of the pulse.
The intensity of the detected backscatter radiation conveys information about the attenuation of the medium through which the pulse is traveling and other properties, such as the local scattering coefficient. This information, in turn, may be used to generate a temperature versus depth (or length) distribution for the region of interest. OTDR techniques may also be used to determine the depth distribution of other quantities, including for example strain, disturbance, magnetic fields. The elapsed time may be converted to distance based on the knowledge of the group velocity of light in the medium; and thus, the elapsed time and the distance along the fiber may be referred to interchangeably.
For relatively long ranges, OTDR may encounter challenges due to the relatively wide dynamic range of the signals that are received by the reflectometer. The specific dynamic range for some reflectometers may reach 40 decibels (dB) one way and beyond (depending on fiber type, resolution and many other parameters), which translates to a variation of the optical power in the backscatter signal by eight orders of magnitude, or 160 dB in electrical dB terms. The long length of the optical fiber typically results in a strong attenuation in the propagation to and from the point of interest, and in the case of distributed sensors, the possible variation of the signal strength that results from the measurement itself adds to the dynamic signal range.
In a conventional OTDR system, an optical-to-electrical device, such as an avalanche photodiode (for example), converts the received optical energy into a current. A preamplifier converts the current from the photodiode into a voltage that then undergoes further processing by the rest of the system (further amplification, filtering and analog-to-digital conversion, as examples). At the lower end of the dynamic signal range, the noise of the preamplifier sets a minimum signal level that can be detected above the noise floor for a given degree of post-processing (signal averaging, for example). At the higher end of the dynamic range, the strongest allowable signal is the one that generates enough current to cause the preamplifier output to swing to, but not exceed, the preamplifier's maximum output voltage.
Although signal equalization downstream of the amplifier may help improve the linearity of the analog-to-digital conversion process, such equalization does not solve the fundamental problems that are set forth above. In addition, for the case in which the optical source is a narrow-band device, such as is the case in coherent optical time domain reflectometry, the dynamic signal range increases dramatically, such as by a factor of one hundred, for example.
Thus, there exists a continuing need for better ways to control a dynamic signal range in an OTDR system.